Porous inorganic solids have found great utility as catalysts and separations media for industrial applications. The openness of their microstructure allows molecules access to the relatively large surface areas of these materials that enhance their catalytic and sorptive activity.
Amorphous and paracrystalline materials represent an important class of porous inorganic solids that have been used for many years in industrial applications. Typical examples of these materials are the amorphous silicas commonly used in catalyst formulations and the paracrystalline transitional aluminas used as solid acid catalysts and petroleum reforming catalyst supports. The microstructure of the silicas consists of 100-250 Angstrom particles of dense amorphous silica, with the porosity resulting from voids between the particles. Since there is no long range order in these materials, the pores tend be distributed over a rather large range. This lack of order also manifests itself in the X-ray diffraction pattern, which is usually featureless.
Paracrystalline materials, such as certain aluminas, also have a wide distribution of pore sizes, but tend to exhibit better defined X-ray diffraction patterns, usually consisting of a few broad peaks. The microstructure of these materials consists of tiny crystalline regions of condensed alumina phases, with the porosity of the materials resulting from irregular voids between these regions. Since, there is no long range order controlling the sizes of pores in the material, the variability in pore size is typically quite high. The sizes of pores in these materials fall into a regime called the mesoporous range which, for the purposes of this application, is from about 2 to about 50 nanometers (nm).
In sharp contrast to these structurally ill-defined solids are materials whose pore size distribution is very narrow because it is controlled by the precisely repeating crystalline nature of the materials' microstructure. These materials are called “molecular sieves”, the most important examples of which are zeolites. Certain zeolitic materials are ordered, porous crystalline aluminosilicates having a definite crystalline structure as determined by X-ray diffraction, within which there are a large number of smaller cavities that may be interconnected by a number of still smaller channels or pores. These cavities and pores are uniform in size within a specific zeolitic material. Since the dimensions of these pores are such as to accept for adsorption molecules of certain dimensions while rejecting those of larger dimensions, these materials are known as “molecular sieves” and are utilized in a variety of ways to take advantage of these properties. The precise crystalline microstructure of most zeolites manifests itself in a well-defined X-ray diffraction pattern that usually contains many sharp maxima that serve to uniquely define the material. Similarly, the dimensions of pores in these materials are very regular, due to the precise repetition of the crystalline microstructure. Molecular sieves typically have pore sizes in the microporous range, which is usually quoted as 0.2 nm to less than 2.0 nm, with a large pore size being about 1.3 nm.
More recently, a new class of porous materials has been discovered and has been the subject of intensive scientific research. This class of new porous materials, referred to as the M41S materials, may be classified as periodic mesoporous materials, which include an inorganic porous crystalline phase material having pores larger than known zeolite pore diameters, for example, diameters of 1.5 to 30 nm. The pore size distribution is generally uniform and the pores are regularly arranged. The pore structure of such mesoporous materials is large enough to absorb large molecules and the pore wall structure can be as thin as about 1 nm. Further, such mesoporous materials are known to have large specific surface areas (e.g., 1000 m2/g) and large pore volumes (e.g., 1 cc/g). For these reasons, such the mesoporous materials enable reactive catalysts, adsorbents composed of a functional organic compound, and other molecules to rapidly diffuse into the pores and therefore, can be advantageous over zeolites, which have smaller pore sizes. Consequently, such mesoporous materials can be useful not only for catalysis of high-speed catalytic reactions, but also as large capacity adsorbents.
The preparation of periodic mesoporous materials typically requires that the film be spun on to the substrate. Spin on processes have disadvantages that include, for example, the physical dimensions of films that can be prepared by the process. Although thin films are desired for certain applications, spin on processes typically result in a film thickness of at least 100 nm. Thus, there remains a need for new methods of preparing periodic mesoporous films.